The purpose of this study is to investigate the influence of voluntary inhibition of movement, using either negative motor imagery or Go/NoGo reaction task, on cortical inhibitory mechanisms. Intracortical inhibition (ICI) and silent period (SP) are two major cortical inhibitory mechanisms demonstrated by transcranial magnetic stimulation (TMS). Alterations in these inhibitory mechanisms have been extensively studied in various situations, but the influence of voluntary inhibition has not been elucidated yet. In normal volunteers, therefore, we plan to determine if voluntary inhibition of movement influences these cortical inhibitory mechanisms. The primary outcome measures would be any changes in motor evoked potential (MEP) size and intracortical inhibition (ICI) parameters.